PEF meters are now seen to be of clinical value in the diagnosis and management of asthma and they are widely prescribed for this purpose in a portable form suited to domiciliary usage. This last form typically involves a piston-and-cylinder assembly through which the piston is movable against a spring bias by the action of a forced expiration, with the piston position affording a measure of the expiratory flow. A well known meter of this form is described in UK Patent Specification No. 1,463,814.
In any event, notwithstanding the value and usage of PEF meters, work leading to development of the present invention indicates that they have shortcomings and are open to improvement. More specifically their operation is effort-dependent. As a consequence of this, successive measures taken over a period of time by the same subject are not necessarily usefully comparable, and comparability between measures taken from different patients is probably less reliable.